There are many types of mechanical couplings between parts. In one variety, a first part is retained within a second part. In one form, the first part may be a tool and the second part may be a tool holder. In another form, the first part may be a retaining pin, and the remaining parts may be at least two components fixed relative to each other by the retaining pin.
One type of mechanical coupling between parts includes a spring-loaded ball, in a recess formed in the second part, that engages a dimple or depression formed in the first part. When the first part is inserted into a complementary shaped recess formed in the second part, the engagement of the ball of the second part in the dimple of the first part prevents relative movement between the two parts, and in particular may prevent the first part from slipping out of the recess in response to vibration of, or shock applied to, the first and/or second parts.
A disadvantage of this type of mechanical coupling is that it requires the formation of a bore in one part to receive the ball and spring for urging the ball from the bore, and the formation of a dimple or depression in the complementary part. Further, such a mechanical coupling may limit the ability of the second part to be oriented in multiple positions relative to the first part. Moreover, the amount of retaining force a ball-and-dimple mechanism can provide may be limited.
Another type of mechanical coupling includes a set screw, which may be threaded through the second part to engage and retain the first part when the first part is placed within a void or recess in the second part. The set screw contacts the first part in order to retain the first part within the void of the second part. A disadvantage of using a set screw is that shock and/or external vibration may gradually work the set screw loose and the first part may become decoupled from the second part. In some applications, the first part may include a depression to receive an inner end of the set screw. This also may limit the ability of the second part to be oriented in multiple positions relative to the first part. Furthermore, the set screw of the second part may tend to damage the surface of the first part. In addition, the retaining force delivered by the set screw may be insufficient.
Yet another type of a mechanical coupling is an interference-fit coupling between parts. In any interference-fit coupling, the shank is slightly larger than the receiver before the coupling is made. One form of an interference fit is a shrink fit, in which, to compensate for the size difference between the shank of the first part and the receiving opening of the second part, the material surrounding the receiving opening in the second part is heated, and/or the complementary shaped shank of the first part is chilled prior to insertion thereof into the receiving opening. Normalization of the temperature of the first and/or second parts creates an interference fit between the parts. Another type of interference-fit coupling is a press fit, in which the size of the opening of the second part is slightly less than the size of the shank or insertion portion of the first part. The shank is then forced into the opening, which causes the opening to expand slightly and/or the shank to compress slightly. In the case of an interference fit, such as a shrink fit or a press fit, the inherent elasticity of the materials making up the first and the second parts provides the normal forces and the resulting frictional forces, causing the two parts to engage each other.
An example of the use of such interference fits is an end mill that is retained within a tool holder. By providing an interference-fit mechanical coupling, the cylindrical body of the shank of the tool may be retained within the tool holder in a variety of orientations.
However, an-interference-fit coupling may be susceptible to loosening when subjected to shock or vibration. For example, since frictional forces are produced only due to the inherent elasticity of the parts, which is purely a material property, vibrational forces, especially those directed normal to the axis of the shank, create directional elastic deformation of both interfering surfaces (e.g., the surface of the shank and the wall of the hole or recess within the tool holder). When there is an axial component to the force acting on the shank, corresponding points of the external surface of the shank and the inner wall of the recess may lose contact with one another and the tool may effectively “walk” out of the recess in the tool holder, as the directional elastic deformation progresses about the circumference of the shank.